The field of imaging is replete with references describing various mechanisms and methods for transporting media through an imaging apparatus. Many of the transport mechanisms include provisions for properly aligning or deskewing individual sheets of media as they travel through the mechanism.
Several prior art patents teach the concept of creating a buckle in a sheet of media to correct for media skew. An exemplary patent disclosing this concept is U.S. Pat. No. 5,543,909 for TWO STEP, LARGE LATITUDE, STALLED ROLL REGISTRATION SYSTEM (the '909 patent). The '909 patent teaches a media feed apparatus for deskewing a media sheet and adjusting the timing of the feed of the sheet through the printer. A media sheet 110 is advanced substantially horizontally along a paper path by a preregistration nip 153. As shown in FIG. 1 of the '909 patent, the sheet lead edge is over-driven into a stalled registration nip 159 until a deskew buckle is formed in the sheet. The spring force of the deskew buckle coupled with the gravitational force pulling the sheet and buckle downwardly tends to urge the sheet lead edge toward the registration nip 159 and correct any skew in the sheet.
After the deskew buckle is formed in the '909 patent, the registration nip 159 is activated and the sheet 110 is fed through the registration nip a fixed distance until the sheet lead edge trips a downstream sensor 160. If paper feed timing adjustments are required, the registration nip 159 is again stopped and the preregistration nip 153, which is driven continuously, feeds a trailing portion of the sheet toward the registration nip 159, thereby creating a timing buckle in the sheet. As shown in FIG. 2, the timing buckle can be much larger than the deskew buckle. The size of the timing buckle is limited by a movable buckle chamber member 155. After a predetermined time, the registration nip 159 is activated and the sheet 110 continues along the paper path.
It is also known in the art to deskew a sheet lead edge by rotating in reverse a roller assembly that forms a nip. One prior art patent that discloses this concept is U.S. Pat. No. 5,427,462 for a METHOD AND APPARATUS FOR PAPER CONTROL AND SKEW CORRECTION IN A PRINTER (the '462 patent). This patent teaches advancing a sheet of media into a nip and slightly therebeyond. The rollers are then rotated in reverse, causing the lead edge of the sheet to jiggle or dance within the nip and settle parallel with the nip under its own weight.
While the apparatus described in the '909 and '462 patents assist in deskewing media, they also contain several drawbacks. For example, when a sheet of media is conveyed substantially upwardly and a buckle is formed, gravity tends to pull the buckle and the leading edge of the sheet downwardly and away from a registration nip. Similarly, if the sheet is conveyed around a corner, the buckle spring force is directed towards and substantially absorbed by the paper guides defining the corner. In either event, the sheet is either incompletely deskewed or not deskewed at all, and a jam or misfeed may result. This problem is particularly apparent with lightweight media having a low stiffness, in which case the buckle spring force is small to begin with. Thus, a need exists for an improved apparatus for deskewing media that overcomes the drawbacks of the prior art.